If you ride long distances at night in poor conditions, and if you can afford it, the light may be worth it. The light weight and compact battery size means you can use your bottle cages for water, not batteries.

As to quality, I have an HID, but not a $600 one. It greatly increases visability--is that a smudge in the pavement, or a pothole? Is that black ice or water? The HID also lights up the area above the path, so I can avoid low-hanging branches. Vars also give me more room since I got the HID.

Is it worth it for everyone? No. But it helps keep me safe.Spending an additional $400 on a light is far more valuable to me than an upgrade to Ultegra or Campy components.